Supply - Situation

[Mowgli]

Hi! 

In my recent PBEM, me and my opponent had a supply situation we couldn't quite understand. We are still learning, so maybe you can help us out? :)

Here is a picture of the situation (Sicily 43, alternative "Gruppo Mobile E" scenario):



The Americans (green counters) try to get supply from the beach ("supply sources") over the bridges ("bridges") into Gela ("supply destination").

The Italians (yellow counters) try to cut the supply line with their tank units. Now, the Italians have all routes within their ZoC. However, the Americans have a chain/corridor of units running through the Italian ZoC (green X), so the supply should get through? But in the game, all units in Gela are isolated (except for the Rangers, who are immune as they're "commando" units).

• One idea is that the HQ-unit (the green X low right) is the weak link for the Americans? The manual states that HQ-units don't have ZoCs, but from my understanding this does not automatically imply that the unit itself can't "lift" enemy ZoCs? 
  
• And even if the HQ-unit is the weak link, there must be another weak link, as there is another supply source one hex to the west of the HQ-unit! This would indicate that the artillery unit (ordinary field arty) is the weak link?
  

Any help to clear the situation up is much appreciated!